1. Field of the Invention
The invention is generally related to the area of optical communications. In particular, the present invention is related to optical wavelength multiplexing or add/drop devices with high reflection channel isolation and the method for making the same in compact size.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high capacity capabilities. Wavelength division multiplexing (WDM) is an exemplary technology that puts data from different sources together on an optical fiber with each signal carried at the same time on its own separate light wavelength. Using the WDM system, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. To take the benefits and advantages offered by the WDM system, there require many sophisticated optical network elements.
Optical add/drop and multiplexer/demultiplexer devices are those elements often used in optical systems and networks. For example, an exchanging of data signals involves the exchanging of matching wavelengths from two different sources within an optical network. In other words, an add/drop device can be advantageously used for the multi-channel signal for dropping a wavelength while simultaneously adding a channel with a matching wavelength at the same network node. Likewise, for transmission through a single fiber, a plurality of channel signals are combined via a multiplexer to be a multiplexed signal that eventually separated or demultiplexed via a demultiplexer.
A fundamental element in add/drop devices and multiplexer/demultiplexer is what is called a three-port device. As the name suggests, a three-port device has three ports, each for a multi-channel signal, a dropped or added signal or a multi-channel signal without the dropped or added signal. FIG. 1A shows a typical design of a three-port add/drop device 100. The optical device 100 includes a common (C) port 102, a reflection (R) port 104, and a transmission (T) port 106. When the device 100 is used as a multiplexer (i.e., to add a signal at a selected wavelength λK to other signals at wavelengths other than the selected wavelength λK), the T-port 106 receives a light beam at the selected wavelength % K that is to be multiplexed into a group of beams at wavelengths λ1, λ2, . . . λN excluding the selected wavelength λK coupled in from the C-port 102. The R-port 104 subsequently produces a multiplexed signal including all wavelengths λ1, λ2, . . . λK, . . . λN.
Likewise, when the optical device 100 is used to demultiplex signals, the C-port 102 receives a group of signals with wavelengths λ1, λ2, . . . λK, . . . λN. The T-port 106 produces a signal with the selected wavelength λK while the R-port 104 subsequently produces a group of signals including all wavelengths λ1, λ2, . . . λN except for the selected wavelength λx. In general, the optical paths towards a R-port and a T-port are referred to as R-channel and T-channel, respectively.
FIG. 1B shows an exemplary internal configuration 110 of the optical device 100 of FIG. 1A. As shown in FIG. 1B, there is a first GRIN lens 112, an optical filter 114 (e.g., a multi-layer thin film filter) and a second GRIN lens 116. In general, a dual-fiber pigtail is provided in a holder 118 (e.g., a dual-fiber pigtail collimator) and coupled to or positioned towards the first GRIN lens 112, and a single-fiber pigtail is provided in a second holder 120 and coupled to or positioned towards the second GRIN lens 116. Essentially the two GRIN lenses 112 and 116 accomplish the collimating means for coupling an optical signal with multi channels or wavelengths in and out of the C port 102, the R port 104, or the T port 106. In general, the three-port device 100 is known to have a very low coupling loss from the C-port to both the R-port and the T-port for use as a demultiplexing device, or vise versa as a multiplexing device.
FIG. 1C shows an application 120 of fiber-to-the-home (FTTH) using a three-port device. A downstream signal 122, namely a multiplexed signal carries essentially two individual light signals at wavelength 1490 nm and 1550 nm. The signal 122 is coupled from a pigtail fiber to a first thin film filter 124 that is configured to reflect a light signal at wavelength 1550 nm. When the signal 122 impinges upon the filter 124, the light signal at wavelength 1550 nm is thus redirected to a lens 126 that focuses the signal onto a laser sensitive component 128 (e.g., a photodiode). The laser sensitive component 128 converts the light signal to an electronic signal for further processing.
On the other hand, some of the signal 122 transmit through the filter 124 and essentially carries the light signal at wavelength 1490 nm. The 1490 nm signal impinges upon a second thin film filter 130 that is configured to reflect a light signal at wavelength 1490 nm. As a result, the 1490 nm signal is redirected to a lens 132 that focuses the signal onto a laser sensitive component 134 (e.g., a photodiode). The laser sensitive component 134 converts the light signal to an electronic signal for further processing.
At the same time, an upstream signal is at wavelength 1310 nm and emitted from a laser emitting device 136. The upstream signal is focused by a lens 118. As the wavelength of the upstream signal differs from the selected wavelength for the filter 124 or 130, the upstream signal thus goes through both of the filters 130 and 124 and is subsequently coupled to the pigtail fiber for transmission.
FIG. 1D is another representation of FIG. 1C, where it shows the use of one 3-port device (filter) together with a dual-band BOSA (bi-directional optical sub-assembly) to interface with three separate opto-electronic interfaces. FIG. 1E shows a cascade of two 3-port devices (one tri-band and one dual-band) to offer a 4-port passive component to deal with the three interfaces. While the approach A has a better integration structure, the approach B may offer overall better device optical performance and thereby can offer better overall signal quality.
The configuration in FIG. 1C, 1D or 1E operates based on the implementation 120. In principle, the implementation 120 works well in a bidirectional module for multichannel use to separate two or three multiplexed channel signals. However, a careful study of the implementation 120 reveals some problems in practical applications. One of the problems is the isolation between the two separated signals. It will be shown below that the separated signals interfere with each other. In other words, one signal carries a residual or a small portion of another signal. Another one of the problems is the efficiency of the separated signals. Because of the residual of one signal leaking into another, there is a loss to the signal, which can be significant when two signals are different in intensity.
It is well known that a frequency response of a thin film filter depends on an incident angle of a signal impinging upon the filter. When the incident angle is small, the frequency response of the thin film filter is maintained. When the incident angle is large, especially as large as 45° angle used in the implementation 120, the frequency response of the thin film filter is severely degraded. Noticeably, a slope region of the frequency response becomes substantially increased. The slope region, also referred to as a deadband, is the region between a stopband and a passband. In applications of separating channel signals or demultiplexing a multiplexed signal, the deadband is desirably as small as possible such that adjacent channel signals in proximity can still be cleanly separated.
For many fiber optic telecommunication applications, such as the fiber to the home (FTTH) application, as illustrated above, the wavelength separation between two channel signals (e.g., the 1550 nm signal and the 1490 nm signal) is often close to 50 nm. Using a uncooled laser for transmission for the purpose of reduced cost, the band separation of the two downstream signals can be as narrow as 30-40 nm. It consequently requires the deadband of a thin film filter no more than 30-50 nm. With an incident angle as large as 450, it is very difficult to the implementation 100 to achieve a desired separation of such two channel signals. When the channel signals can not be satisfactorily separated, interferences among channel signals could take place. In the implementation 100, the 1550 nm signal and the 1490 nm signal could interfere with each other. It may be worse when a weaker signal is interfered by a small portion of a stronger signal, sufficiently enough to cause distortions or unrecoverable loss of the weaker signal.
Accordingly, there is a great need for techniques for providing high isolation from the T-channel channel such that the errors or residuals to the R-channel are minimized. The devices so designed are amenable to small footprint, broad operating wavelength range, enhanced impact performance, lower cost, and easier manufacturing process.